The present invention relates to a vehicle brake device and a method of controlling the vehicle brake device, and more particularly, to a regenerative cooperative brake device that includes hydraulic brake means and regenerative brake means and a method of controlling the regenerative cooperative brake device.
In the past, a so-called regenerative cooperative brake device, which applies a braking force to a vehicle by a hydraulic brake force generated according to the driver's operation of a brake pedal and a regenerative brake force generated by a regenerative brake device, has been known as a vehicle brake device. An object of a regenerative brake is to convert the kinetic energy of wheels during braking into electric energy and to effectively use energy. However, the maximum regenerative brake force has a limitation due to the speed of a vehicle during braking or the charge state of a battery.
For example, a so-called brake-by-wire control system where a hydraulic circuit and a brake pedal are mechanically separated from each other has been employed as such a regenerative cooperative brake device. However, the brake-by-wire control system has a complicated structure and is expensive in comparison with an existing in-line control system where a hydraulic circuit and a brake pedal are fluidically connected to each other.
As for an in-line regenerative cooperative brake device, JP-A-2006-96218 proposes that a loss-stroke is provided on a brake pedal at the time of early braking in order to improve regeneration efficiency, a regenerative brake force increased to the maximum value of a regenerative brake during the loss-stroke, and hydraulic pressure is then generated. Specifically, an operating rod of the brake pedal is provided with an operating force transmission mechanism that does not transmit an operating force of the brake pedal to a master cylinder during a predetermined state from the start of the pressing of the brake pedal. That is, a second operating rod, which is mounted on a first piston of the master cylinder, is slidably engaged in a cylindrical portion of a first operating rod mounted on the brake pedal. Accordingly, a stroke until the predetermined state from the start of the pressing of the brake pedal is referred to as the loss-stroke in which a pressing force is not transmitted to the second operating rod from the first operating rod, and only a regenerative brake force is applied in the loss-stroke. Further, if a regenerative brake force is insufficient until the predetermined state from the start of the pressing of the brake pedal, a pump for an ABS control system is driven so that hydraulic pressure is forcibly generated in wheel cylinders.
However, the improvement of a brake feeling is not sufficient in the structure disclosed in JP-A-2006-96218. In general, it is known that a relationship between pressure in the wheel cylinders and the stroke of the brake pedal is not a completely proportional relationship and a required brake pedal stroke is reduced as the pressure in the wheel cylinders is increased (hydraulic pressure is increased with a small stroke). In the structure disclosed in JP-A-2006-96218, hydraulic pressure in the wheel cylinders is substantially zero when the brake pedal reaches a predetermined state from the start of the pressing of the brake pedal if a sufficient regenerative brake force is obtained in the early stage at the time of the braking. However, if a regenerative brake force is insufficient, hydraulic pressure is forcibly generated in the wheel cylinders by the drive of the pump as described above. For this reason, when the brake pedal reaches a predetermined state from the start of the pressing of the brake pedal, hydraulic pressure is already present in the wheel cylinders. Accordingly, the operational feeling of the brake pedal after the predetermined state varies between a case where a regenerative brake force is sufficient and a case where a regenerative brake force is insufficient, in the early stage at the time of the braking
Further, it is premised in JP-A-2006-96218 that the maximum regenerative brake force is constant. However, actually, the maximum regenerative brake force changes according to the speed of a vehicle, the charge state of a battery, or the like. Accordingly, a problem of a pedal feeling, when a regenerative brake force is changed after a loss-stroke, that is, while the master cylinder and a reservoir do not communicate with each other, is not yet solved.
Furthermore, a loss-stroke is provided on the operating rod of the brake in JP-A-2006-96218. Accordingly, even though a regenerative brake force is insufficient at the early stage of braking when an ABS control system fails, hydraulic pressure cannot be generated. As a result, there is a concern that a case where a braking force is not generated at all may occur. Moreover, when a booster disposed between the brake pedal and the master cylinder fails, the generation of hydraulic pressure is delayed by a loss-stroke although a braking force is less effective. For this reason, the braking force is far less effective.
JP-T-2007-500104 discloses means for making brake fluid escape into a low pressure accumulator when performing braking by a regenerative brake and pumping brake fluid from the accumulator by a motor pump unit, which is to be electrically controlled, when increasing hydraulic pressure, as means for obtaining a comfortable brake feeling. However, when brake fluid escapes into the low pressure accumulator in JP-T-2007-500104, a discharge valve is opened while hydraulic pressure is present in the wheel cylinder. The discharge valve is usually on/off controlled. Accordingly, when the discharge valve is opened under pressure, significant vibration and rough sounds are generated.